1. Field of the Invention
The present invention relates to a video image signal band compressing system for a digital video tape recorder (D-VTR) of a high picture quality.
More particularly, the present invention relates to a video image signal band compressing system which digitally converts a component image signal as a signal source and compresses the digitally converted signal in a unit of fixed length depending upon a human visual property and a statistical nature and frequency distribution of the image to execute a additional high speed playback of the D-VTR.
2. Description of Related Art
With a recently advanced technique in the fields of a computer, a semiconductor device and a digital signal processing system, a storage and transfer of a digitized signal is also developed remarkably.
When such an image signal is stored or transferred in digital, a massive amount of the image signal must be processed as compared with an analog data, so compression of the image data is needed to effectively utilize a capacity of a storage medium and a data transferring channel. Accordingly, it requires that the image data must be compressed in a given amount while completely representing original information contents. Further, the image data compressed has a reliable characteristic to an error occurred in a storage medium and a hardware for compressing in image data must be easily embodied.
According to a typical image signal band compressing system, the image signal is divided into several blocks and the blocked image signal is subjected to a variable length coding achieved by way of a transform coding technique, for example, a discrete cosine transformation (DCT). In the image signal coding using such a DCT technique, since energy of the image signal is condensated on a low frequency portion, the original information can be reliably represented using a small quantity of data by coding the low frequency region of the image signal.
According to the aforementioned variable length coding technique, a codeword of given data is variably assigned with reference to the probability density of the image data and the codeword having variable length is transferred. This technique is suitable to apply when the image data to be processed are not uniformly distributed.
Therefore, the variable length coding technique assisted by the DCT coding can be effectively employed to reliably express the ununiformely distributed image data.
According to the technique denoted above, however, only local property of the image is considered in the image data coding. When the complex image signal having a fluctuating region and a flattening region of the image data is subjected to the previously noted coding in the D-VTR which compresses the image signal into a target amount of data in unit of a fixed length, and transfers and stores the compressed image signal, surplus data produced in processing the flattening region are not sufficiently utilized, so that serious deterioration of the picture quality in a portion adjacent to the flattening region and the fluctuating region is occurred undesirably.
In the variable length coding of the image data, it is required to a constant amount of data per a unit time duration in order to store or transfer the data coded. Accordingly, a buffer is needed to temporarily store the coded data and a step size of quantization must be controlled by a buffer pre-control or buffer post-control technique to adjust the amount of the data produced.
While such a buffer pre-control technique has an advantage that an error in the storage medium is propagated in a unit of fixed data length, it is difficult to adjust the amount of the data produced in a unit of fixed length to the desired amount of data.
On the contrary, according to the buffer post-control technique, such a step size in the quantization is decided in accordance with the repleted degree of a buffer under periodic confirmation of the status of the buffer. Accordingly, the amount of the data to be stored or transferred is coincided with the desired amount of data. Plus it has a block effect occurred due to a quantization error when the fluctuating region adjoins the linear region.